In many complex modern radio networks and systems there is a need to establish the locations of radio units or stations in the network. This need can arise in a stationary radio unit, such as a base station, or in a mobile radio unit such as a subscriber unit. For example, in the case of a subscriber unit, location information is used for routing of incoming calls to the subscriber unit, handing off of the subscriber unit from cell-to-cell, controlling power levels and for other purposes. Location information for a base station is used in similar ways.
With the progress of technology from analog cellular radio systems to modern digital cellular radio systems, as well as LANs and specialty radio systems such as military systems, the distinction between a mobile unit and a base station can become blurred. Subscriber units may be fixed and may act as base stations for other subscriber units. Base stations may be mobile.
Proposals have been made for inclusion of a global positioning system (GPS) receiver in each base station, or even each subscriber unit. As the cost of a GPS receiver decreases, this additional feature becomes more attractive, but as the cost of a base station also decreases, there remains a need for a manner of providing location information without additional hardware cost in the system.
Existing methods of measuring of the distance of a subscriber unit from a base station include using a sub audible tone (SAT) which is looped around in an analog subscriber unit in phase with a received coherent SAT. The base station measures the phase shift between the transmitted SAT and the returned SAT to determine the distance of the subscriber unit from the base station. Another method is used in a global system for mobile communications (GSM) system in which the timing of the synchronization word received at a base station is measured relative to a fixed time base at the base station and timing advance is used to instruct the mobile unit to advance or retard its timing so that transmission bursts sent by the mobile unit are received within the correct slots and the base station. Such an arrangement effectively establishes a number of propagation distances between the mobile unit and the base station.
Existing methods for measuring distance between a mobile unit and a base station are very coarse and merely place the mobile unit at a certain radial distance from the base station.
There remains a need for an improved method of establishing location information in a radio network.